Cathode-ray tube with magnetic compensating means



Aug. 5, 1947. c. L. RICHARDS 2,425,125

CATHODE RAY TUBE WITH MAGNETIC COMPENSATING MEANS Filed March 22, 1943 INVENTOR C'MUDFLAN 0 RIC/ ARM HZ mm,

ATTORNEY Patented Aug. 5, 1947 CATHODE-RAY TUBE WITH MAGNETIC COMPENSATING MEANS Claude Langdon Richards, Little Haldon, New digate, Surrey, England, assignor to Hartford National Bank and Trust Company, Hartford,

Conn., as trustee Application March 22, 1943, Serial No. 479,999 In the Netherlands July 2t, 194i) 6 Claims. i

This invention relates to a device comprising a cathode ray tube in which use is made of a magnetic field parallel to the axis of the tube for the concentration of the cathode ray beam and of at least one deflecting coil comprising an iron core.

The magnetic field parallel to the axis of the tube which is required for the concentration of the cathode ray beam is frequently produced by a coil arranged axiall of the tube to surround it. If, a coil having a non-rotation-symmetrical iron core is used for the deflection a deformation of the concentrating field may occur in the proximity thereof. This results in astigmatism or generally speaking, in improper definition of the image reproduced.

According to the invention, this disadvantage is obviated by arranging a body of ferromagnetic material to surround the tube and to be substantially normal to the axis thereof so that the influence of the deformation of the concentrating field brought about by the iron core is substantially balanced.

In order that the invention may be clearly understood and readily carried into effect, it will now be described more fully with reference to the accompanying drawing in which Fig. 1 is a diagrammatic view of one form of construction of a device according to the invention, I designating a cathode ray tube of normal construction whose neck 2 contains a usual device 3 for roducing a cathode ray beam. To concentrate the beam, a coil 4 is arranged to surround the neck 2 coaxially of the tube and is supplied from a source of direct current (not shown). vertical deflection of the cathode ray beam is brought about by means of a two-part deflecting coil 5 comprising a bipolar iron core 6, of which a section on the line A-A is shown in Fig. 2. The horizontal deflecting means are not shown in Fig. 1; they may in the usual manner comprise a second deflecting coil or a set of electrodes for electrostatic deflection.

A magnetic field is produced by the coil 4 parallel to th axis of the tube. By reason of the nonrotation-symmetrical shape of the iron core 5 (Fig. 2) the field of the coil 4 (a concentration field) is subjected to deformation adjacent the core 6 and this results in astigmatism or, generally speaking, in improper definition of the image. According to the invention, to avoid this provision is made preferably between the coil 4 and the core 6 of a small plate I of the same shape as the core, but placed in a position rotated by 90 relatively thereto to surround the neck 2 and to be substantially normal thereto, as is shown in Fig. 3. Thus, the lack of symmetry which is due to the core '6 is substantially balanced by the plate 1. As a matter of fact, it is true that the plat I has much less iron material than the core 6 but the former is nearer to the coil 4; it can thus be ensured that the influence of both on the field or" the coil l, insofar as its action on the cathode ray beam is concerned, is about the same. The place of the plate I surrounding the neck 2 must be chosen accordingly; by the position rotated by relatively to the core 6, it is ensured that the influence of the combination on the field of concentration is practically the same as of a single core without a plate lblll} of rotation-symmetrical shape. As a matter of fact, in the case of efiicient arrangement, such a rotation-symmetrical core does not bring about improper definition or astigmatism of the image.

If the number of pairs of poles of the core is it instead of 1 as shown in Fig. 2, the desired result may be obtained by the provision of a plate 7 whose shape is similar to that of the core, said plate consequently comprising as many pairs of poles. The plate must, however, be rotated relatively to the core through an angle of 360/4n.

In th presence of a second iron core, for exampie, for the horizontal deflection, the influence of this core is also to be taken into account when determining the shape and the position of the plate I, the best course being in this case ascertainment thereof by experiment.

According to the invention, the plate I may be used in addition to adjust the position of the image received. This may be effected by displacing the plate in its own plane in a vertical or a horizontal direction. A slight alteration of the concentrating field is thus brought about, which results in a displacement of the image in the direction in which the plate 1 is moved.

Having described my invention, what I claim is:

l. A device comprising a cathode ray tube in which means are present to produce a magnetic field parallel to the axis of the tube for the concentration of the cathode ray beam, and deflection means are present comprising at least one deflecting coil comprising an iron core, at least one body of ferromagnetic material being arranged to surround the tube and to be substanti ally normal to the axis thereof so that the influence of the deformation of the concentrating field brought about by the iron core or cores is substantially balanced.

2. A device as claimed in claim 1, wherein said core presents polar regions wherein said body of 3 ferro-magnetic material Whose section normal to the axis of the tube is of the same shape as that of the core thereby presenting like polar regions is arranged to surround the tube in the proximit of the core, the body being rotated about the axis of the tube relatively to the core through an angl of 360 divided by four times the number of pairs of polar regions of the core.

3. A device as claimed in claim 1, in which the concentrating field is produced by means of a concentrating coil coaxial of the tube, and wherein said body of ferro-magnetic material is in the form of a small plate arranged between the core and the coil, the thickness of the plate being small relatively to the dimension of the core in the direction of the tube axis.

4. A device as claimed in claim 1, wherein the body of ferromagnetic material is a plate which can be displaced in its own plane in horizontal and vertical directions to secure optimum balance.

5. A beam concentrating and deflecting system for a cathode ray tube comprising means for providing a beam concentrating field, means in the form of a member of magnetic material having polar regions for providing a pulsating beam deflecting field, and means for compensating for the distorting effect of the means providing the deflecting field on the concentrating field, said last named means being in the form of another magnetic member having regions corresponding to the polar regions of said first named member of magnetic material but angularly displaced therefrom.

6. A beam concentrating and deflecting system for a cathode ray tube comprising means for providing a beam concentrating field, means in the form of a member of magnetic material having polar regions for providing a pulsating beam deflecting field, and means for compensating for the distorting efiect of the means providing the deflecting field on the concentrating field, said last named means being in the form of another magnetic member having polar regions angularly displaced from the polar regions of said first named member of magnetic material, said second named member being displaced from said first named member along concentrating field, the amount of said displacement being determined in accordance with the ratio of the quantity of magnetic material in each of said members.

CLAUDE LANGDON RICHARDS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,981,322 Nakajima et a1 Nov. 20, 1934 2,177,688 Cawein Oct. 31, 1939 2,185,138 I-Ianns-Heinz Wolfi Dec. 26, 1939 2,188,579 Schlesinger Jan. 30, 1940 2,212,206 Holst et a1 Aug. 20, 1940 2,259,233 Tingley Oct. 14, 1941 Re. 22,009 Farnsworth Jan. 20, 1942 2,297,407 Gunther Sept. 29, 1942 

